The first two things you notice when you walk into Bill Gates’s private office, just outside Seattle, are a wall-size installation of the periodic table with a sample of each chemical element in its own glass-fronted vitrine and handsome bookshelves that reach nearly to the ceiling.

Science and the written word, Mr. Gates’s twin passions.

In 1975, Mr. Gates founded Microsoft with Paul Allen, and the two quickly built it into the dominant force in the personal computing software industry. It ultimately became the world’s largest software company and made Mr. Gates, for a time, the richest man in the world. (Bloomberg currently estimates his wealth at $95 billion. Jeff Bezos, with an estimated net worth of $114 billion, holds the top spot.)

But, in a surprising turn roughly two decades into his tenure at Microsoft, Mr. Gates shifted his attention — and ultimately more than $30 billion of his fortune — to philanthropy, creating with his wife the Bill and Melinda Gates Foundation. (The couple married in 1994; the foundation was founded six years later.) Reportedly the largest private foundation in the nation, the Gates Foundation works to improve health in developing countries and expand opportunities for advancement among those least advantaged in the United States.

In addition to focusing on his philanthropic work, Mr. Gates, 62, is an avid reader, who frequently comments on books on his popular Gates Notes blog. (Recent recommendations include “Evicted,” by Matthew Desmond, and “Believe Me,” by the British comic Eddie Izzard. “If you’ve never seen Eddie perform his stand-up routine,” Mr. Gates wrote, “you’re missing out.”) And when Mr. Gates puts in a good word for a title, it often sells. In a 14-tweet thread aimed at recent college graduates last May, for instance, Mr. Gates praised “The Better Angels of Our Nature: Why Violence Has Declined,” by Steven Pinker, calling it, in one of those 14 tweets, “the most inspiring book I’ve ever read.” By the next morning, “Better Angels,” which had been published six years earlier, was at the top of Amazon’s best-seller list.

On a recent afternoon, I waited in Mr. Gates’s office, admiring its striking view of Lake Washington and Mount Rainier in the distance, where I was joined by Mr. Pinker, 63, a professor of psychology at Harvard University and the author of 10 books, two of them Pulitzer Prize finalists.

Mr. Gates quickly joined us. The men first met in 1995 when Mr. Pinker visited Microsoft to talk about his book “The Language Instinct,” about the evolution of language for human communication. The pair have been speaking regularly ever since — most recently about Mr. Pinker’s forthcoming book, “Enlightenment Now,” which makes the case for science and reason as forces for steady and measurable human progress and was recently reviewed by Mr. Gates in his blog.Continue reading the main storyRelated Coverage

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Over a lunch of pizza and salad, on what turned out to be the day before the recent government shutdown (“I’m still an optimist, but what’s that all about?” Mr. Gates asked), the two men discussed the surprising arcs of their careers, the work that lies ahead to ensure continued human progress and the challenge of creating the perfect toilet.

As we began our conversation, Mr. Gates readily acknowledged that the person he is today is not one he would have recognized when he was in his 20s and single-mindedly building Microsoft. “I was a zealot,” he said. “I didn’t believe in weekends. I didn’t believe in vacations. I knew everybody’s license plate so I knew when they were coming and going. That was my life: doing great software.”

Philip Galanes I was shocked when you left Microsoft for your foundation.

Bill Gates When you get married and have kids, as I did in my 30s and 40s, there’s a certain normalcy in working on multiple fronts. Plus, you don’t want a tech company run by somebody in their 60s. At least I didn’t want to. I ended up retiring at 53. But because of the foundation work I started when I was 45, I was part time at Microsoft by then, learning about global health, vaccines, Africa.

PG But why a foundation? Why not luxurious vacations?

BG Well, the question for us was: Given this wealth and our value system, what’s the highest-impact way to give it back? Melinda and I chose global health as our biggest issue, and U.S. education as our second one. And we’ve surrounded global health with agriculture and sanitation, family planning, antismoking. So, it’s a broad definition.

PG Do you see your career as a progression toward being a fuller human being?

BG Now you’re putting a value judgment on it. I won’t endorse that or not endorse it. But for a young man in his 20s, writing software night and day may be the best way to add to human welfare. I’d never heard of vaccines. I didn’t have any money. But the personal computer, the internet, hey, that’s what I was good at. And I enjoyed doing it every day.

PG Steven, there’s an elegant parallel between your career and Bill’s — starting with language and continuing to human well-being.

Steven Pinker It’s interesting that I sort of followed Bill’s trajectory and ended up in a similar place by accident. We were both interested in language when we met: digital communication systems, logic, programming. But I’m a psychologist, not a linguist. My interest grew out of interest in human nature. Language is what makes humans most obviously different from other animals. And in “The Language Instinct,” I suggested that language was a Darwinian adaptation that allowed us to cooperate and communicate.

Later, Bill moved toward evidence-based philanthropy, and I wrote “Better Angels,” an evidence-based take on history with a surprising-to-some outcome that we’ve made a lot of progress. I came across statistics that homicide rates in the Middle Ages were about 35 times what they are today in Europe. When I posted this online, I started receiving correspondence citing more examples: The rate of death in warfare has come down by a factor of 20 since 1945. Domestic violence is down. Child abuse is going down. I was sitting on all these data sets showing reductions in violence that few people were aware of that I thought ought to be better known.

BG I was stunned by “Better Angels” because I was coming around to the same view: That “things getting better” is the greatest story that no one knows.

PG The motto of the foundation is: “Every life has equal value.” And in your new book, Steven, there’s the idea that we can’t want something good for ourselves without wanting it for everyone. But in truth, I want better things for my husband and my kids than for you. Is that evil, or human?

BG That’s natural. It’s even predicted by evolutionary logic. What makes Papua New Guinea — where there’s no police and revenge after revenge — different from Western society is that when we give ourselves over to the law, we want it to be executed impartially. We gain stability. But if you could get your son off, of course you’ll try.

SP You left off a crucial piece in framing the proposition, Philip — which comes from Spinoza. He said those under the influence of reason desire nothing for themselves that they do not desire for all humankind. But reason is not a powerful part of human nature. Innately, we favor family over strangers, our tribe over other tribes. It’s only when we’re called upon to justify our beliefs — not consult our gut feelings, but convince others of the right way to act — that we conclude that all lives have equal value.

PG Steven’s new book tells us we’re living longer, with greater wealth, peace and equality, and less racism and sexism. It’s so counterintuitive to the feeling in the air. What’s the value in correcting that? Will it change behavior?

BG Absolutely. The present techniques we use as a society — law, democracy, some degree of progressive taxation — as imperfect as they are, are working by lots of key measures. So, when you consider a radical change, like “Hey, let’s tear up the global trade agreements; they’re a disaster,” you’re more likely to implement it if you think things are getting worse. “Let’s tear up the treaties. Let’s try a nondemocratic approach.” Your willingness to go off the current path is much, much higher. But you can hardly be serious about government — or society as a whole — unless you say: What are the measures that count, and how are we doing on those measures?

SP There’s a tendency in journalism and political debates to assume that it’s easy to achieve a perfect society: “Good people would do that.” The fact that we don’t means that evil people must be running the system: “Let’s throw them out and find nobler ones.” This leads to empowering charismatic despots and destroying institutions that have done a lot of good. But we have no right to expect perfection. We should appreciate how much better off we are and try to improve our institutions guided by what works and what doesn’t. I’m sure Bill gets this all the time: “Why throw money at the developing world? They’re just going to have more babies and be just as poor.”

BG “It’s always been bad, always will be bad.”

SP It’s just not true.

PG But can’t overstating problems energize us in terms of solving them?

BG There’s a paradox in letting yourself be very, very upset about what remains to be done. What indicator improves even faster than reduction in violence? Our distaste for violence. We’re more upset about it today. If I see someone spanking a kid — I’m stealing from Steven’s book — I might get up and say: “Hey, wait a minute!” Forty years ago, it might have been more like: “Do you want to borrow my belt?” There are parts of the world that are still like 40 years ago. But to read Steven’s book and think it says, “Don’t worry, be happy,” is to misread it. Because seeing the world through the eyes of that poor kid ideally wants to make you give some money, even though there are many fewer such kids than 50 years ago.

SP You can say the same fact two ways. Extreme global poverty has been reduced from 90 percent 200 years ago to 10 percent today. That’s great! Or you can say: More than 700 million people in the world live in extreme poverty today. They’re the same fact, and you have to be able to describe them to yourself both ways.

PG Bill, did your success at creating Microsoft make you more optimistic about tackling big issues at the foundation?

BG Absolutely. Take the foundation’s toilet project. We want to reinvent the toilet so it doesn’t need water piped in or out — just a chemical process, so that even Indian cities that will never spend $1 billion can have a toilet as good as a Western one. This is a 10-year quest. If I didn’t have the success I had at Microsoft, I would never have the bullheadedness to embark on this project.

PG Your mathematical skills probably make the foundation run differently than most?

BG You also have to understand science and history, and how to pick the right people to be able to back the right projects. Having optimism about science and feeling in command of scientists, that’s like what I did at Microsoft. And, yes, I embrace more risk. Most philanthropists don’t take huge, 10-to-15-years-type risks.

SP That whole story is an example of where actual moral benefit and human moral intuitions are not in sync. The person who invents an affordable and efficient toilet should be made a saint. Think how much human happiness will be granted, how much human suffering eliminated. We should think quantitatively; it’s the morally enlightened way. But it’s not the way our brains evolved when we make moral evaluations.

PG Especially now. People are feeling left behind and angry about it. Wage stagnation, #MeToo, Black Lives Matter, access to health care. Is this the right moment for a book that says, “Generally, things are looking up”?

BG There are different time frames. Take the #MeToo movement. We’re in a period of awakening now. It is not worse today than it was five years ago. We’re just more aware. And five years from now this outrage will have been a factor in making things better than five years ago, when it was hidden.

PG But I was asking about the tribalism of the moment, whether your devil is the 1 percent or the bad hombres. Can science and reason really unbundle tribal thinking?

SP One of the biggest enemies of reason is tribalism. When people subscribe to an ideology, they suck up evidence that supports their preconceptions and filter out evidence that goes against them. Contrary to the belief of most scientists that denial of climate change is an effect of scientific illiteracy, it is not at all correlated with scientific literacy. People who believe in man-made climate change don’t know any more about climate or science than those who deny it. It’s almost perfectly correlated with left-wing versus right-wing orientation. And a move toward greater rationality would unbundle them and let evidence inform what the optimal policies ought to be.

PG But breaking deadlock surely requires more than asking people to be rational.

BG I don’t think we’ve come to the point yet where democracy and science is going to stop working to improve lives. I was at a climate conference in Paris, and, honestly, there were as many nonscientific things said there in a good cause as I imagine were said at the non-climate convention that I didn’t happen to attend. But I’m optimistic. I do think awareness of how things have worked is important to recreate a conservative center — that is, make us careful about what we change.

PG And here I thought you were going to say: Innovation drives progress, not Washington.

BG But the problem now is that innovation is not viewed as an unalloyed way to improve the human condition. And that’s fair, because it’s not pure. Does social media split us into tribes in a way that’s dangerous? Does it create, even in high school social circles, a channel for bullying, or a desire to look perfect in photos? Is A.I. going to proceed so quickly that work, which is something people worship, will suffer bad distributional effects, and people won’t know what to do? This is an unfortunate time for saying, “Take all your damn negative thoughts, and I’ll innovate away from them.” People are seeing difficulty with that argument.

SP There’s also a dichotomy between the roles of innovation and policy. There are certain things that governments are always going to do better than private innovators. Basic research, for instance. Also, governments can do things that might be necessary to accelerate the development of innovations. And in terms of numbers, the Gates Foundation is smaller than government by a significant margin, right?

BG Our total spending is $5 billion. Foreign aid is $30 billion.

SP We have to resist the temptation to say that the fantastic good that’s done by foundations and innovators takes governments off the hook.

BG All three sectors — philanthropy, which is about 2 percent of the economy; government, which is about 30 percent; and the private sector, which is the balance and largest by far — each have their role.

PG Name a problem we may think of as intractable that you’re optimistic about solving in the near future.

SP War between countries. Civil wars are harder to eliminate because there are so many insurgent and militia forces. But there are only 192 countries. They could agree not to declare war on each other. I think we’re on the way.

BG Disease elimination is something that runs against people’s general pessimistic outlook. It’s amazing to think that polio might be gone. That one is pretty near.

SP How about malaria?

BG We’re working hard on that.

SP Like zeroing it?

BG Between now and 2025, if things go well, we’ll be down to very tough places, like equatorial Africa. But we’ll get there.

PG Last question: Since Oprah’s near presidential announcement at the Golden Globes and speculation about Mark Zuckerberg’s political aspirations, have people asked you to consider getting involved in political public life?

BG Well, I am involved. But I won’t be a candidate.

PG Leaving the way clear for Steven?

SP Wonderful!

BG It’s tricky. You hope that as one party moves in one direction, the other party doesn’t move too far in the opposite direction. We’ll see. There haven’t been that many anti-elitists, anti-internationalists elected president. But we have one now.